The Bureau of Alcohol, Tobacco, Firearms and Explosives is working to recover from another much-investigated incident. The scandal grew out of an ATF effort to stop gun smuggling into Mexico.

LINDA WERTHEIMER, HOST:

Years later, authorities face the same basic problem. Guns are mostly banned in Mexico.

INSKEEP: But they're mostly legal in the United States and it's hard to stop some U.S. gun buyers from smuggling weapons across the border. Here's NPR's Ted Robbins.

TED ROBBINS, BYLINE: I'm standing north of the border with Mexico near Nogales, Arizona, surrounded by hills and mesquite trees and cactus, and a canyon. In that canyon, border patrol agent Brian Terry was shot in December 2010. He was shot by Mexican bandits with an AK-47. That gun was traced back to a botched U.S. operation called Fast and Furious, one of several operations which had let the guns go from American dealers to drug cartels in Mexico unchecked.

BERNARD ZAPOR: I'm holding this shortened version of the AK-47.

ROBBINS: Bernard Zapor is the ATF agent in charge of the Phoenix field division. He is showing me a weapon similar to the one used to kill Agent Terry. It's a favorite with the cartels.

ZAPOR: In Mexico they shoot the AK-47 round, but it's something that you can hold in your hands and easily conceal.

ROBBINS: Zapor took charge of this office just last fall. He says there's no chance the ATF is still doing operations like Fast and Furious.

ZAPOR: First off, we've had a wholesale leadership change from the directors to the executive staff to my contemporaries in the field.

ROBBINS: He knows the Mexican cartels are still getting U.S. guns, largely the way they did five years ago, using straw purchasers, people who can pass a federal firearms background check, legal buyers fronting for criminals. But Zapor says they're not buying 50 or 100 guns at a time as they used to.

ZAPOR: They'll buy five at one place, five at another place, five at another place, thinking that that is going to somehow obscure the obviousness of their intent.

ROBBINS: The ATF knows what's happening because it has a new tool. Gun dealers in the four border states - Texas, New Mexico, Arizona and California - have to report when anyone buys more than one rifle at a time. Former ATF supervisor Mark Jones says there's a problem with that. The problem is the reporting requirement only applies to the border states.

MARK JONES: It'll slow the traffic down in those four states because they don't want the kind of scrutiny that comes with that so they're going to other places that they don't have to have that scrutiny. And until you put that regulation in place nationwide, you're going to see that displacement.

ROBBINS: A nationwide regulation would meet stiff opposition from the gun industry. Its trade association fought the border state reporting requirement before a judge upheld it. The gun industry has also successfully stopped any attempt at a gun sale database. Sales records are kept by dealers who may not be inspected for years.

Bottom line, the only time ATF knows where a gun is is after its used in a crime. Now Customs and border protection is trying to stop guns flowing south. Here at the Port of Entry in Nogales, Arizona, CBP officers knock on a car's gas tank. They're listening for telltale signs of hidden guns or hidden money from drug sales. CBP says southbound inspections are done in what's called a pulse and surge fashion.

That means intermittently. The records CBP gave us show it seized between 83 and 242 weapons a year across the entire border. David Shirk is a research with the University of San Diego. He co-authored a study on firearms trafficking across the border. He says that's a tiny fraction of the guns reaching Mexico.

DAVID SHIRK: At best you can hope for maybe five, 10 - catching five, 10 percent of what's actually moving. And that's kind of our best estimate of how effective interdiction is with regards to drugs moving north.

ROBBINS: Think about it. The government spends billions of dollars a year trying to stop drugs coming north. It spends nothing close to that trying to stop guns and drug money going south. David Shirk says the U.S. ought to be doing more.

SHIRK: So when our neighbor's house is on fire, and it's our gasoline that's causing it to burn, I think we have both a moral imperative, but also a certain self-interest in making sure that we resolve that problem.

ROBBINS: The ATF has put more agents in the Southwest. It's uncertain how much they or any of these other measures are slowing the flow of weapons across the border. Ted Robbins, NPR News.

INSKEEP: Now, you recall that NPR News took a road trip along the border. We told stories of crossing the border, including people like the writer Oscar Casares, who crossed with a bit of fear.

OSCAR CASARES: I got these looks, like are you serious? Are you going across? And I got...

INSKEEP: It's like 100 feet.

CASARES: I know.

INSKEEP: Yet when we crossed, we found the two sides had a lot in common. It's a region we call the Borderland. Having heard the voices, you can now see the images. Today we publish Borderland, an online magazine - stories, pictures, maps, history, statistics on Borderland, a different experience of a distinctive place. You can find it at NPR.org. This is NPR News. Transcript provided by NPR, Copyright NPR.